Clinical studies document the benefits of exercise both in the general population and in heart failure (HF) patients. Undoubtedly systemic effects of exercise play an important role in these benefits. However, we hypothesize that exercise initiates salutary signaling mechanisms within the heart itself that also contribute and could be exploited to mitigate HF. To identify cardiac transcriptional components differentially regulated by exercise, we used a recently developed platform to assess expression of all ~2000 transcriptional components in the mouse genome in models of early physiological (exercise-induced) and pathological (pressure overload-induced) cardiac hypertrophy. The initial candidates identified C/EBP and CITED4, provide support for our overall hypothesis and are the focus of this application. Our preliminary data demonstrate that the transcription factor C/EBP is specifically downregulated with exercise, while expression of CITED4 is increased. In contrast, neither changes in early pathological hypertrophy induced by transverse aortic constriction (TAC). In vitro studies demonstrate that a reduction in C/EBP is sufficient to drive an increase in both cardiomyocyte size and proliferation, as well as increased CITED4 expression. Reduced C/EBP2 expression in vivo in heterozygous germline knockout mice resulted in phenotypes similar to those seen with endurance training, including improved exercise capacity as well as increased cardiomyocyte size and proliferation. Heterozygous C/EBP knockout mice also showed increased CITED4 expression comparable to that seen with exercise, and were resistant to cardiac dysfunction after TAC. The overall goal of this proposal is to understand the intersecting roles of C/EBP and CITED4 in the heart. We now propose to develop unique in vivo models that will enable us to determine whether the phenotypes observed reflect cell autonomous effects of these transcription factors in cardiomyocytes as well as the potential of these pathways to mediate protection and/or cardiomyocyte proliferation in fully adult hearts. We will also investigate the downstream mechanisms responsible for the phenotypes observed. Understanding the pathways that confer the cardiac benefits of exercise may provide new insights into physiological mechanisms controlling cardiomyocyte growth and proliferation as well as a foundation for novel therapeutic approaches in heart failure and other cardiac diseases.